One type of adhesive that finds extensive commercial utility, particularly in packaging applications, is the hot-melt adhesive, which is a material that can be heated to a melt and then applied to various substrates such that a bond is formed upon cooling and resolidification. A wide variety of thermoplastic polymers, particularly ethylene-based polymers, such as ethylene-vinyl acetate copolymers (“EVA”) have been proposed for use in hot-melt adhesives but often the adhesive formulation requires substantial use of low molecular weight ingredients such as wax and tackifier to adjust the viscosity and glass transition temperature to useful ranges for hot melt adhesive applications. Where the polymers have substantial crystallinity and hence a poor compliance to substrates, this can significantly reduce their potential for use in low temperature applications. On the contrary, amorphous polymers often have poor cohesive strength and so require the presence of large concentrations of high molecular weight materials to improve cohesive strength but this leads to high melt viscosity and poor processability. Also desirable in packaging is to have adhesive compositions that exhibit a sufficiently long time before hardening to preserve sufficient adhesion (known in industry as “open time”) in assembly operations such as box closures, yet a quick enough setting speed to allow shortest time application of adhering pressure (known in industry as “setting time”).
Although many of the alpha-olefin copolymers currently used in adhesive compositions are derived predominantly from ethylene (see, for example, International Patent Publication Nos. WO98/03603 and WO99/24516), copolymers derived predominantly from propylene have also been proposed for use in adhesive compositions. For example, U.S. Pat. No. 3,954,697 discloses a single component, hot-melt, pressure-sensitive adhesive based on propylene/higher 1-olefin copolymers containing 40-60 mole % of the higher 1-olefin. Suitable comonomers include 1-hexene, 1-heptene, 1-octene, 1-nonene and 1-decene. The copolymer has a melt viscosity range at 190° C. of 10,000 cp to 75,000 cp, a density of 0.85 to 0.86, a glass transition temperature of −30 to −45° C., and has no melting point measurable by Differential Scanning Calorimetry. Thus the copolymer is amorphous with no residual crystallinity or crystallinity of a very low order. The copolymer is produced using a Ziegler-Natta catalyst. It is to be appreciated that a hot melt pressure sensitive adhesive (HMPSA) is an adhesive that is applied hot to a substrate to form an article, such as a tape or label, which is subsequently used at ambient temperature to initiate adhesion.
U.S. Pat. No. 6,627,723 discloses a poly-alpha olefin copolymer comprising (a) from 60 to 94 mol % of units derived from one alpha mono-olefin having from 3 to 6 carbon atoms, preferably propylene; (b) from 6 to 40 mol % of units derived from one or more other mono-olefins having from 4 to 10 carbon atoms and at least one carbon atom more than (a), preferably butene-1, hexene-1 or octene-1; and (c) optionally from 0 to 10 mol % of units derived from another copolymerizable unsaturated hydrocarbon, different from (a) and (b), preferably ethylene; wherein the diad distribution of component A in the inter-polymer as determined by 13C NMR divided by the calculated Bernoullian diad distribution is less than 1.07; and wherein the storage modulus G′ of the copolymer determined on cooling, measured at 1 Hz, intersects 3×105 Pa at an intersection temperature of less than 85° C. The copolymer is produced by copolymerizing the monomers in the presence of a single site metallocene catalyst, preferably a bridged chiral bis-indenyl metallocene catalyst. The copolymer is largely amorphous and is said to be useful as a hot melt adhesive, both with and without the addition of tackifying resins.
U.S. Pat. No. 6,747,114 discloses an adhesive composition comprising a semi-crystalline copolymer of propylene and at least one comonomer selected ethylene and C4 to C20 α-olefins having a propylene content of greater than 65 mole percent; wherein the copolymer has a weight average molecular weight (Mw) from about 15,000 to about 200,000; a melt index (MI) from about 7 dg/min to about 3000 dg/min as measured by ASTM D 1238(B); and a weight average molecular weight/number average molecular weight ratio (Mw/Mn) of approximately 2. In Example 4, propylene is copolymerized with between 11 and 19 wt % 1-hexene at a temperature of 76° C. to 90° C. over a catalyst comprising dimethylsilandiylbis(2-methyl-4-phenyl-1-indenyl)zirconium dimethyl activated with dimethylanilinium-tetrakis(pentafluorophenyl)borate to produce semi-crystalline copolymers having a melting temperature, Tm, between 93° C. and 107° C., a heat of fusion, ΔH, between 58.5 and 96.6 J/g and a melt viscosity at 190° C. of between 2230 and 66,000 cps. Pressure sensitive hot melt adhesives are formed by blending 30 wt % of each copolymer with 50 wt % of Escorez® 5380 tackifier and 20 wt % of Kaydol Oil and the resulting adhesives exhibit a tensile strength in excess of 127 psi, an elongation in excess of 368% and a Shear Adhesion Failure Temperature (SAFT) to Kraft paper of 74° C. to 83° C. In addition, for the copolymer formed from 19 wt % 1-hexene, a hot melt adhesive is formed by blending 80 wt % of the copolymer with 15 wt % of Escorez® 5380 tackifier and 5 wt % of Kaydol Oil. The resulting adhesive exhibits a tensile strength of 1452 psi, an elongation of 768% and a Shear Adhesion Failure Temperature (SAFT) to Kraft paper of 102.5° C. All of the adhesives formed had at least 20% of the other ingredients due to the high molecular weight of the starting copolymers, and had a long set time.
International Patent Publication No. WO 2004/046214, published Jun. 3, 2004, discloses a polyolefin adhesive comprising a polymer having a Dot T-Peel of 1 N or more on Kraft paper, an Mw of 10,000 to 100,000, a branching index (g′) of 0.95 or less measured at the Mz of the polymer and a heat of fusion of 1 to 70 J/g, wherein the polymer comprises 50 weight % or more of an alpha-olefin having 3 to 30 carbon atoms, and optionally up to 50 weight % of a comonomer selected from the group consisting of ethylene, butene, hexene, octene, decene, dodecene, pentene, heptene, nonene, 4-methyl-pentene-1,3-methyl pentene-1,3,5,5-trimethyl-hexene-1, and 5-ethyl-1-nonene, and up to 10 weight % of a diene. The polymer is preferably produced using a mixed catalyst system comprising a first transition metal compound capable of producing a crystalline poly-alpha-olefin, preferably isotactic or syndiotactic polypropylene, having a crystallinity of 20% or more, and a second transition metal compound capable of producing an amorphous poly-alpha-olefin, preferably atactic polypropylene, having a crystallinity of 5% or less.
In particular, Example 71 of WO 2004/046214 discloses the production of a copolymer of propylene, hexene and 1,9-decadiene by feeding propylene (14 g/minute or 81 wt %), hexene (3.29 ml/minute or 15.8 wt %) and 1,9-decadiene (0.206 ml/min) to a liquid filled, single-stage continuous reactor using a mixed metallocene solution catalyst system comprising dimethylsilylbis(2-methyl-4-phenylindenyl)zirconium dimethyl and dimethylsilyl-(tetramethylcyclopentadienyl)(cyclododecylamido)titanium dimethyl at a temperature of 115° C. The product, comprising both amorphous and semi-crystalline propylene copolymers, had a Tm of 89.8° C., a Tc of 42.6° C., a glass transition temperature, Tg, of −15.2° C., a heat of fusion of 27.0 J/g and a viscosity of 524 cp at 190° C. and, when used as an adhesive without additives, exhibited a set time of 3.5 seconds and 80% fiber tear at 20-25° C. using a file folder as a substrate. When formulated with 2 wt % of Escorez® 5637 tackifier and 5 wt % of Paraflint H1 wax, the copolymer of Example 71 provided an adhesive with a set time of 2 seconds and 90% fiber tear at 20-25° C. on a file folder substrate. However, the formulated adhesive would have no fiber tear and poor adhesion at low temperature (−18° C.) on difficult substrates such as Inland paper board.
As illustrated by the prior art cited above, the copolymers derived from either ethylene or propylene currently used in hot melt adhesives are of high molecular weight. However, high molecular weight copolymers exhibit poor processability and require high concentrations (>20%) of other low molecular weight additives, such as wax and tackifier, to reduce the adhesive viscosity to the applicable range as well as to improve other adhesive properties. It would therefore be desirable to develop adhesives using low molecular weight copolymers having the desired end-use viscosity. However, low molecular weight polymers often have poor cohesive strength. There is therefore a need for copolymers which exhibit low viscosity together with a combination of good adhesion at low and high temperatures and acceptable mechanical strength.
Certain novel propylene copolymers have been produced that, when combined with small quantities of wax and/or a functionalized polyolefin, exhibit an excellent balance of adhesive properties, including a good low temperature (−18° C.) adhesion performance, short set time and a high toughness at relatively low application viscosity.